Need help for configure Git Large File System (git-lfs)

I'm Unity 3D game developer. I'm using Bitbucket from the last 2 years for saving my codes online. I have numbers of repositories over there.

A Few days back, while check-in, I was getting an error that my repo has reached the size limit of 2GB. Then, to resolve the error I came to know that Bitbucket has the limit of 2GB. I was also looking for the paid service but I was not able to find out. Then again I made a new repo and checking the same project & with initial check-in the size of repo was 1.3 GB.

Then I came to know that I need to enable the Git large file for the resolution to such problems. But even struggling from the last few weeks I'm not able to enable that.

Now can you guys please help me to configure that. I need step by step instructions for that. I tried lots of methods with shell/terminal, but hard luck.

P.S.: I'm not interested/familiar with the terminal commands for check-in /checkout. I always use Source tree. It helps me to setup any repo from any provider like bitbucket, Github, GitLab etc. So kindly suggest me the source tree steps.